There is no one-size-fits-all solution for sustainability, and mining waste streams are increasingly becoming the target of sustainability practices in the industry.
Sustainability in the industry can take on many forms and encompass applications across the entire production chain.
Sustainable practices for industry waste streams have been gaining popularity in recent years, including repurposing tailings and treating mine water for reuse, as well as recycling off-the-road tyres. Sustainable practices can also be applied to waste streams indirectly created by industry.
An example of this is coal ash, a byproduct of burning coal at coal-fired power plants. With more than 1.2 billion tonnes of coal ash produced annually, work to address the challenges of coal waste’s substantial impact to the environment is an industry focus.
Tackling this challenge, a team of researchers at RMIT University has developed a low-carbon concrete that has the potential to significantly enhance recycling of coal wastes and sustainability in construction.
As well as tackling the copious amounts of coal ash, this development also works to mitigate the high carbon embodiment from cement production, which constitutes around eight per cent of global emissions.
RMIT’s new prototype material can reclaim double the amount of coal ash compared to current standards, which halves the cement usage for concrete production and maintains exceptional performance.
Dr Chamila Gunasekara holds a sample of the low-carbon concrete. Image: Michael Quin, RMIT
Project lead from RMIT’s School of Engineering, Dr Chamila Gunasekara, said, “Our research aims to make a vital contribution to Australia’s net zero emission target towards 2050 and pave the way for a new generation of building and infrastructure construction, both in Australia and internationally.”
The team behind the scenes
Led by Dr Gunasekara, the project on low-carbon concrete development commenced in 2016, soon after the completion of Dr Gunasekara’s PhD candidature.
Dr Gunasekara began experimenting on various supplementary cementitious materials to reduce carbon embodiment of concrete production alongside a team of researchers at RMIT University.
The team recognised the need to address sustainability concerns in building and construction and, with concrete still the most used material within the sector, approached the problem with low-carbon concrete.
Cement clinker is the most common ingredient in concrete and was flagged as a key area where sustainability could be enhanced.
“With decades of research and development history, the challenge in further enhancing sustainability still lies in clinker production, a crucial step in cement manufacturing, where CO2 reduction has already been optimised,” Dr Gunasekara said.
“While further reduction of carbon footprint in clinker production is difficult, there is potential in the emergence of alternative sustainable materials. However, for these materials to be viable in engineering practice, they must be readily available with a robust supply chain in given regions.”
This way of thinking shifted the attention of the project to the various ash wastes generated from coal-fired power plants. Even with energy structure reform towards an increasing adoption of renewable resources, traditional power stations are expected to persist for the next decade, resulting in an accumulation of under-utilised coal-fired byproducts for years to come.
“There are plenty of such materials in Australia and around the globe. Take fly ash as a prime example, which will remain abundantly available despite the shift towards renewables in the future,” Dr Gunasekara said.
Materials science and low-carbon concrete
Fly ash is a byproduct from coal combustion which has gained prominence due to its fine granular particle size, as well as pozzolanic properties, which could enhance the durability and strength of concrete when used to partially replace cement clinker.
The usage of coal fly ash in the construction industry is prevalent in recent decades. Coal fly ash has been widely introduced to cement manufacturing as a valuable supplementary cementitious material to create beneficial synergy with cement during hydration.
Since the initial adoption of fly ash as a supplementary cementitious material around the globe, research and innovation have led to a deeper understanding of its properties over time, driving its widespread application in the construction sector.
Alongside an increase in environmental concerns (specifically global warming), the demand for sustainable building practices has intensified and the use of fly ash as a cement alternative has attracted even more attention.
This is due to cement production being a major source of CO2 emissions, primarily caused by the calcination process, where limestone (calcium carbonate) is heated to produce lime (calcium oxide), releasing CO2 in the process. Furthermore, fossil fuels, including coal, oil and natural gas, are often used as energy sources in cement kilns for the calcination process.
Cement manufacturing is energy-intensive, requiring large amounts of electricity for grinding raw materials into the cement clinker, further contributing to greenhouse gas emissions and exacerbating climate change.
To this end, the RMIT team developed a nano-engineered method to substitute up to 80 per cent of the cement in concrete with coal fly ash, surpassing the typical 40 per cent replacement rate in existing low-carbon concretes.
The RMIT team: (L-R) Dr Yuguo Yu, Professor Sujeeva Setunge, Dr Dilan Robert, Dr Chamila Gunasekara and Dr David Law. Image: Michael Quin, RMIT
“Our addition of nano additives to modify the concrete’s chemistry and microstructure allows more fly ash to be added without compromising engineering performance. That is how we doubled the current cement reduction in concrete. Our research entailed considerable time and effort in refining the optimal mixture to accommodate this doubled waste inclusion in concrete,” Dr Gunasekara said.
Beyond just fly ash, the team at RMIT has demonstrated that lower-grade pond ash from coal slurry storage ponds can also be repurposed effectively. Concrete prototypes that have been made using both fly ash and pond ash have been shown to meet Australian engineering and environmental standards, exposing the potential to repurpose a vastly under-utilised resource from coal mining and power generation.
“It’s exciting that our preliminary results show similar performance for concrete prepared with lower-grade pond ash, potentially opening a whole new hugely under-utilised resource for cement replacement. Compared to fly ash, pond ash is under-exploited in construction due to its different physical and chemical characteristics.
“There are hundreds of megatonnes of ash wastes sitting in dams around Australia, and much more globally. These ash ponds risk becoming a major environmental hazard and the ability to repurpose this ash in construction materials at scale would be a massive win across many sectors,” Dr Gunasekara said.
“With this new low-carbon concrete demonstrating comparable structural capacity and durability to conventional concrete, we are currently conducting a full lifecycle assessment to understand the environmental impact of this material from production to end of life.
“It is expected that this innovation can reduce CO2 emissions by ten to 20 percent, which is a significant achievement. After developing these mix proportions as well as matching the chemistry and other factors, the next step was to test the long-term performance, such as in 25 years etc., with the assistance of the computer tool developed by Dr Yuguo Yu.
“In the end, we need to meet all specifications and standards to satisfy engineering and environmental requirements to put into engineering practice,” Dr Gunasekara said.
Assessing long-term performance
Virtual computational mechanics expert at RMIT University, Dr Yuguo Yu, got involved in the project, confronting a persistent challenge in civil engineering – predicting the long-term durability of newly developed low-carbon materials.
Dr Yu said the team developed a predictive modelling tool for assessing the time-dependent performance of various cementitious materials and composites.
“We’ve now created a physics-based model to predict how the low-carbon concrete will stand the test of time, which offers us unique opportunities to reverse engineer and optimise mixture design from computational insights,” Dr Yu said.
The development of the modelling tool was a big undertaking, with Dr Dilan Robert and Dr David Law key members of the team, partnering with AGL’s Loy Yang Power Station and the Ash Development Association of Australia. Dr Yogarajah Elakneswaran from Hokkaido University also contributed to the development of the model.
With this joint effort, the developed computer modelling program enabled the team to forecast the new concrete’s long-term performance, in line with the goal to reduce general cement usage and address environmental waste challenges, whilst promoting sustainable construction practices with enhanced long-term durability and resilience.
The modelling tool can also enable the optimisation of concrete mixes by understanding the interactions of each ingredient over time, enhancing material density and compactness with nano additives.
Circularity in mining
Given that ash wastes generated by coal-fired power plant originate from coal mining, creating circularity within the mining industry has appeal, especially within the context of Australia.
There are many structures supporting mining processes that can be built from cementitious materials, such as acid mine cut-off walls and tailings dams.
The use of fly ash and/or pond ash can improve the strength, durability and impermeability of concrete, making it an excellent material for constructing cut-off walls. The pozzolanic property of ash waste helps in the neutralisation of acidic environments, making it suitable for use in acid mine drainage prevention structures. Additionally, the fine particles of fly ash can refine the pore structure of concrete, reducing permeability to enhance the capability of cut-off walls in preventing seepage of contaminated mining wastewater.
Considering the ready availability of fly and pond ash, and the cheaper price tag than cement clinker, using it can lower the overall cost of auxiliary mining construction projects, creating enormous benefits and circularity within the mining industry.
With appropriate quality control using the new prototype and modelling innovation from the RMIT team, these wastes can be effectively repurposed to enhance the sustainability and safety of mining operations.
“Our priority moving forward is to achieve commercialisation of the low-carbon concrete and computer modelling tool. We have spent a considerable amount of time balancing the chemistry and developing our mixed proportion technology, supported by extensive simulation and physics-based modelling,” Dr Gunasekara said.
“The commercialisation of our work ultimately depends on its reception from end-users.
“We’ve garnered partnerships with many local councils who align with our TREMS research hub. With abundant resources at hand, we’re actively collaborating with local councils to deploy our concrete prototypes with the support of fresh computational insights.”
The team’s low-carbon concrete product is expected to be equally scalable compared to conventional concrete and can be produced in a normal batching plant without needing any additional sophisticated production lines.
“We can take advantage of the existing batching plants for mass production and commercialisation,” Dr Gunasekara said.
Beyond the mining sector, these advancements can also be integrated into a range of construction projects, including footpaths, structural elements in residential buildings and extensions to critical infrastructure like rural bridges and roads, and can have a significant impact on a variety of industries.
Featured image: Boraryn/stock.adobe.com
